The present invention relates to an ignition timing control system for an automotive engine having an exhaust gas recirculation (EGR) system.
In an engine for a motor vehicle, an EGR system is employed to lower the combustion temperature for reducing nitrogen oxides (NOx). The engine having the EGR system is arranged such that the ignition timing is advanced to a maximum extent during EGR operation, in order to prevent the reduction of driveability of the vehicle and the increase of fuel consumption due to the EGR. However, when the requirement of emission control regulation becomes servere, the amount of EGR must be increased to meet the requirement. When the amount of EGR increases, the deteriorations of driveability and fuel consumption can not be avoided by advancing of ignition timing. Accordingly, it has been proposed to cut off the EGR system or to leak the vacuum for operating an exhaust gas recirculation valve to control the recirculated gas so as to render the EGR system inoperative under particular conditions in order to prevent the deteriorations of the characteristics, although the engine operation is in an EGR operative range. However, if ignition timing remains advanced in the inoperative state of the EGR system, the engine is operated at extremely advanced ignition timing, which will cause the occurrence of knocking of the engine to reduce the driveability and increase fuel consumption.